In lots of science fiction works that visit other planets we are often introduced to intelligent plant based lifeforms, be they humanoid looking or potted plant looking.

I'm curious as to what part of the plant has to evolve to gain sapient or near sapient intelligence. Eg, all mammals share an organ known as the brain and central nervous system. What would be the plant based equivalent?

I want to have some plant based organisms but need to figure out what part of the plant is essential to keep and develop. I don't want to accidentally make the most likely area into a vestigial feature (I'm not looking for full sapience but if you can figure that out, I'm sure such an answer would be useful to others)

$\begingroup$This totally made me think of a genius loci D20srd - OGL page. Too close to Sonic's answer so not posting one as my own.$\endgroup$
– KevinOct 17 '16 at 14:09

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$\begingroup$@EveryBitHelps you could get inspired by the movie "Avatar", were plants and animals share a neuronal connector. Your plants could have that same system. Maybe not individually, but more like in "clusters" of plants$\endgroup$
– StrannchOct 17 '16 at 14:55

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$\begingroup$I don't want to give away spoilers, but starting in the second book in the Ender's Game series there are some plants that kinda "house" an intelligence. The last two books in the series actually go into a fair bit of detail about how it is all possible - its very interesting and actually gets quite philosophical. (So if you want a REALLY good read and to see a unique take on it, check out the series :) )$\endgroup$
– BunnyKnitterOct 17 '16 at 22:21

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$\begingroup$@SnyperBunny, totally forgot about the little 'stick lovers'. I always remember the travel time difference aspect when I think of the Ender series. Thanks for the reminder. Time for a refresher!$\endgroup$
– EveryBitHelpsOct 17 '16 at 22:41

6 Answers
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Method A: Evolving a True Plant with Neural Tissue

This method describes how a traditional, plantlike organism could evolve a "brain" while maintaining plantlike characteristics in every generation.

First of all, brains can come in all shapes and sizes!

Jellyfish style: Nothing resembling a brain is present but a distribution of nervous tissue around the body maintains control. Think "reflexes" and not "thoughts".

Cockroach style: Clumps of tissues resembling a brain exist but are spread out in the body.

Spider style: The brain tissue literally spills out of the head and around the body

Human style: One central brain controls the body through a network of tissues

Colossal squid style: The cognetive organ is wrapped around the esophagus in such a way that a large bite of food means permanent brain damage :)

A plant could plausibly evolve and survive with any of these systems in place, but it would likely evolve clumps or spread tissue. This is because plants tend to lose parts, and this method would be most likely to preserve most intelligent thought if said plant were to lose said parts.

Now how would a plant evolve a web of neural tissue?

Well to spread tissue to every part of the plant, it could potentially evolve from a single-celled, photosynthetic organism. As this organism grows larger over time, becoming two or three-celled, one cell mutates to have neuron-like properties. If this three-celled organism can

Sense light with Cell A (photo-receptor, which isn't uncommon and could also be a precursor to the evolution of eyes)

Remember what or where light is with Cell B (precursor to neuron; single neurons are capable of doing simple tasks like this)

Photosynthesize with Cell C (precursor to leaves; must contain chloroplasts or another organelle capable of photosynthesis)

Travel to the light by moving all three cells

Under these conditions, an organism could survive to reproduce, grow larger, and eventually become an organism comprised of two separate, main tissues: "traditional" plant matter, and a web of neural tissue. Eyes could additionally evolve from the original organism's photo-receptors, and these would aid the neural tissue in collecting information.

Method B: Adapting an Organism with a Brain to Photosynthesize

In this method, an organism that is not a plant, but has cognitive capacity, evolves photosynthesis and other plantlike characteristics.

An organism that dwells in sunlight mutates a form of chloroplast, and passes the genes that create them to its offspring. Individuals that spend more time in sunlight will survive to reproduce because they will produce more of their own food (sugars) and will not have to forage or decompose other organisms. Additionally, organisms with larger and more efficient photosynthetic cells or tissues will also survive to reproduce.

What could evolve in this way? Something:

Mobile, to make use of sunlight throughout the day

Intelligent enough to meet the "has a brain" requirement

Relatively flat to make efficient use of sunlight

That can fulfill the chemical needs of photosynthesis for its own system. For example, an organism whose process requires carbon dioxide and releases oxygen would make great use of porous flesh to filter in gases.

That makes contact with the ground, such as a slug, to absorb nutrients from the soil without having permanently anchored roots (for now)

That doesn't contain extremely rigid structures such as bones. Relying on the soil, water, and sunlight won't provide the nutrients to form skeletal structures, so only a soft organism will be able to survive using photosynthesis alone and evolve into a plantlike organism from there.

If you aren't satisfied calling green slugs "plants", here's what comes next: The organisms' metabolisms could slow drastically, or they could find a place such as an open field to proliferate. Either way, they will move less. This is important, as the next stop on our journey is roots!

In order for a plantlike organism to be truly convincing, even if all tissues except its brain now resemble that of a plant, you might want roots. These could plausibly develop if the organism doesn't need to move to find sunlight. The organisms that can reach further into the soil for nutrients while still being stationary in this environment will survive to reproduce, making roots increasingly common and efficient. With the energy these organisms would now have, they could be capable of sprouting upward and forming chutes or stalks. This would put the brain at the base of the plant, between the roots and stems, with all other surviving organs from the original species.

Reproduction for a stationary organism will be problematic, and will likely have to evolve before said organism becomes stationary. One solution could be the development of asexual reproduction, which is not uncommon among plants on Earth. New plants would sprout at the base of parent plants, from the organs that used to facilitate reproduction, but daughter organisms would contain only parent DNA. Another method could be the exchange of genetic information through external contact, such as stems or roots.

Finally, you have a plantlike creature that evolved from another organism through natural selection and a whole lot of unlikely plausibility.

Method C: Symbiosis Between a Brain Species and a Plant

I won't go into much detail on this one, but I will lay a foundation.
Lichens (LIKE-ins) are a symbiotic combination of two or more organisms, always a photosynthetic organism to produce energy, a fungus to anchor to a surface, and sometimes some other things, but let's ignore those other things for now.

Theoretically, your plants with brains could evolve similarly to lichens - a photosynthesizer could produce sugars for a host with a brain, which would in turn house the photosynthetic organism safely.

$\begingroup$I did not know that about cockroaches! No wonder they can survive as a species.$\endgroup$
– EveryBitHelpsOct 17 '16 at 9:54

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$\begingroup$@Zxyrra - That's why squids eat really small bites of food! They kind of fillet the fish they eat. Though I read a paper at least 25 years ago where an octopus had been eating bristleworms. The spines had pierced the octopus's brain, but apparently done no harm to it as it was living a perfect normal life. Normal until someone killed it and dissected its brain and went "Ooh a bristle worm spine!" :-)$\endgroup$
– DrBobOct 17 '16 at 17:05

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$\begingroup$@EveryBitHelps Voila, two more methods B and C as if A wasn't already too complicated and outlandish :)$\endgroup$
– ZxyrraOct 18 '16 at 4:52

$\begingroup$Can you link to what you mean about spiders?$\endgroup$
– JDługoszOct 18 '16 at 20:31

$\begingroup$Feedback should prompt you to edit the post, not just answer in another comment. (Then these comments cqn be cleaned up)$\endgroup$
– JDługoszOct 18 '16 at 21:55

If it is simple reflex movement like Venus flytrap then you can have distributed specialized tissue that control actuators.

But if you want your plant to have more complex behavior (like tracking of prey position before attack) you may need more complex coordination between sensors and actuators. Then you end up with some more centralized system (up to as complex as human brain).

You can actually make in a center of thoughts and senses in middle of overgrown huge plants. If you plan to make xenoplant infestation, that works the best. It can be just simply anywhere, but most probably underground or in a hard crust shell. It may appear after extensive development of the plant, and after stages the plant parts may reorganize to accomodate a nerve center, so you may not be restricted by specific plant part to protect until this stage comes... or if really needed, I would point on the stem right next to where the roots starts.

$\begingroup$Some very interesting reading thanks. I was aware that plants can communicate and even sing, I was wondering if a plant evolved further where the 'brain' equivalent would be. Looks like right now it is all over. I wonder. Does that mean a plant lifeforms behaviour would change if they lost a limb etc? Food for thought.$\endgroup$
– EveryBitHelpsOct 17 '16 at 11:36

I have always been fascinated with the concept of the "hivemind" -a collective of life-forms all operating as a single entity, which I think exemplifies the nature of distributed, plant-based lifeforms. Rather than a single, sapience-inducing part/organ, each life-form has neural-like connections running throughout itself, and either through direct contact or some other construct connects with other lifeforms, forming a large neural net, vastly more capable as a whole than the individual.

This really opens a new set of possibilities that can add new dimension to your world. Even within a hive-mind, you could still have some individualism, e.g. internal factions, discrepancies between regions, etc.

Most plants have specifically evolved (with perhaps the exception of single stalks) to avoid single "parts" or organs on which it relies to survive, so I think a distributed design for your "brain" makes sense here. To answer your question more directly, if you wish to have a neural-heavy or even an "individual intelligence" part of the plant, you could consider the roots of the plant for that purpose- often the most protected part of a plant, and can easily be used for the inter-life-form connections.

This question reminded me of the film Avatar. The flora of the planet Pandora are all linked together to form a large neural network.

Flora on Pandora are of a tropical type several times taller than that existing on Earth. Many, if not all, plant and animal species have bioluminescent properties. The flora specimens seen in the film were all designed by Jodie Holt, a professor of botany at the University of California, Riverside. According to Holt, Pandoran flora are able to communicate with each other through signal transduction and are larger in size than Earth flora due to greater atmospheric thickness, weaker gravity, and stronger magnetism on Pandora. [Source]

Then I looked this up:

Signal transduction refers to the transmission of a molecular signal in the form of a chemical modification by recruitment of protein complexes along a signaling pathway that ultimately triggers a biochemical event in the cell. [Source]

So in the fictional world of Pandora, the plants communicate with each other with chemical signals, forming a planet-wide neural network. So it's less about a single plant evolving a "brain" in this case, and more about the wildlife being linked and able to communicate.